1. Field of the Invention
The subject invention relates generally to a pneumatic switch for fire detection, and more particularly, to a pneumatic detector switch having a single deformable diaphragm for indicating alarm and fault conditions.
2. Description of Related Art
The reliable detection of fire in commercial and military vehicles and aircraft is a critical function. A well-known prior art fire detection system includes a titanium or vanadium wire contained in a pressurized sensor tube. During fabrication, the wire is exposed to high temperature and pressurized hydrogen gas, which it absorbs while cooling. The hydrogen saturated wire is inserted into a sensor tube, pressurized with an inert gas, and then sealed at both ends to form a pressure vessel.
One end of the pressure vessel is then incorporated into a housing that comprises a hermetically sealed and pressurized plenum, wherein pneumatic detector switches are located. When the sensor tube is exposed to high temperature, for example, in the event of a fire or overheat condition in the vehicle, the pressure inside the vessel will rise, impacting the pneumatic detector switches.
Typically, prior art fire alarm systems use two separate pneumatic detector switches, one for indicating an alarm condition and another for indicating a fault condition. The pneumatic detector switches are typically deformable metallic diaphragms that are adapted and configured to move between open and closed switch positions in response to variations in the background pressure within the plenum.
When a deformable diaphragm is employed as an alarm switch, the open switch condition corresponds to a low or normal pressure condition in the plenum, whereas the closed switch position corresponds to a high pressure condition in the plenum. In the open switch position, the diaphragm is not in electrical contact with the alarm circuit. Conversely, in the closed switch position, when there is a high pressure condition in the plenum resulting from a fire or an overheat condition, the diaphragm makes electrical contact with a circuit to activate an alarm.
When a diaphragm is employed as a fault or integrity switch, the closed switch position corresponds to a normal pressure condition in the plenum, whereas the open switch position corresponds to a low or below pressure condition in the plenum. In the closed switch position, the diaphragm makes electrical contact with the circuit to indicate system integrity. Conversely, in the open switch condition, the deformable diaphragm moves out of electrical contact with the fault circuit, indicting a fault condition or loss of pressure within the plenum.
To reduce the manufacturing cost and weight of a fire detection system used in vehicles and aircraft, it would be beneficial to provide a pneumatic detector switch having a single deformable diaphragm for indicating both alarm and fault conditions.